The absorption of light causes a change in the sodium conductance of the photoreceptor cells in both vertebrate and invertebrate eyes. The goal of this research is to elucidate the steps in the process that links the chemical change in a visual pigment molecule to the conductance change in the photoreceptor membrane (photoexcitation). Voltage clamped discrete waves of the ventral photoreceptor will be studied. Discrete waves are single photon responses and their properties follow simple linear rules. The clamping prevents regenerative responses of the photoreceptor membrane. The combined use of discrete wave statistical properties and voltage clamping provides a unique tool with which to study photoexcitation. In addition, microsurgical techniques will be used to insert pipets into the ventral photoreceptor and to perfuse the interior of the call. In this way, precise control of the composition of the interior of the cells will be attained, and it will be possible to introduce macromolecules which may be implicated in photoexcitation. Intracellular perfusion will be combined with the analysis of voltage clamped discrete waves to study photoexcitation on a molecular level.